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问题 实验题

如图所示电路电源两端电压不变，R1是电阻箱，R2是定值电阻。当开关S闭合后，逐步改变电阻箱接入电路的电阻值，根据记录到的电压表与电流表的示数，绘制的图像如图所示。请你根据图像写出电压U和电流I的关系式：U=        。

答案

题目分析：分析电路结构，R1和R2组成串联电路，依据串联电路的电压规律，总电压等于各电阻两端的电压之和。

观察图像，找到2个特殊点，当I=0.1V，的示数为10V，设电源电压为U总，所以有U总=10V+0.1R2

当I=.04A时，的示数为4V，U总=4V+0.4R2；

联立上述两个方程，解出U=12V,R2=20Ω。所以，R1两端的电压U=U总-U1=U总-I

点评：本题较难，在分析电路结构的基础上，结合图像，利用串联电路的规律列方程求出。

选择题

动物对于一个生态系统来说是很重要的，这是因为（　　）

A．动物是食物的唯一来源

B．动物能促进生态系统的物质循环

C．动物能固定二氧化碳

D．动物能释放氧气
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单项选择题

"The first and best of victories is for a man to conquer himself; to be conquered by himself is, of all things, the most shameful", says Plato. Self-control is at the root of all the advantages. Let a man give in to his impulses and feelings, and from that moment he gives up his moral freedom.
A single angry word has lost many a friend. When Socrates found in himself any temper or anger, he would check it by speaking low in order to control himself. If you are conscious of being angry, keep your mouth shut so that you can hold back rising anger. Many a person has dropped dead in great anger. Fits of anger bring fits of disease. "Whom the gods would destroy they first make mad. " "Keep cool", says George Herbert, "for fierceness makes error a fault. "
To be angry with a weak man is to prove that you are not p yourself. "Anger, " says Pythagoras, "brings with folly and ends with regret. " You must measure the strength of a man by the power of the feelings he conquers, not by the power of those which conquer him.
Self-control is man’s last greatest victory.
If a man lacks self-control he seems to lack everything. Without it he can have no patience, no power to govern himself; he can have no self-confidence, for he will always be controlled by his pest feeling. If he lacks self-control, the very backbone and nerve of character are lacking also.

The author’s main purpose in writing this article is to ______.

A．explain that self-control is the key to success
B．teach people how to control everything in order to make a great success
C．distinguish all kinds of self-control and suggest ways for keeping it
D．advise people not to lose temper so as to make and keep more friends
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